GaN-based heterostructures are one of the most promising materials for power switching devices that can contribute to energy saving and can be used as smart power grid interfaces for solar, wind and other renewable energy generations, and traction drives in electric or hybrid electric vehicles. For simplicity and reduction of power loss, normally-off devices are usually required in power switching circuits. Recently, enhancement-mode AlGaN/GaN field-effect transistors (FETs) by thinning AlGaN barrier have been reported. However, it is challenge to achieve simultaneously both positive threshold voltage and higher turn-on voltage for higher power capability with lower power loss. Thus, introduction of gate dielectrics between Schottky metal and the thin AlGaN barrier is inevitable for higher breakdown voltage and higher current capability required for power switching devices. In this work, Al oxide formed by atomic layer deposition (ALD) is used because of its large dielectric constant, large bandgap, and high breakdown field. For a 15-nm thick ALD Al oxide, the results show that the reverse leakage current density (J<SUB>REV</SUB>) of Ni/as-deposited ALD-Al oxide/AlGaN/GaN MOS diodes is lower by up to one order of magnitude than Ni/AlGaN/GaN Schottky diodes. However, the MOS-diodes with as-deposited ALD-Al oxide exhibit even higher current at a low reverse and forward bias (−3 V to 1 V), suggesting that there is probably an unintentional current path along the oxide films or the Al oxide/AlGaN interface. To study this, pre- and post-treatments ALD Al oxide are investigated to modify properties of ALD-Al oxide/AlGaN interface. AlGaN/GaN samples after ohmic metallization are pre-treated before ALD Al oxide deposition in three different methods: 1) zero bias O<SUB>2</SUB> plasma oxidation in an inductively-coupled plasma (ICP) system, 2) high-pressure O<SUB>2</SUB> plasma oxidation in a reactive ion etching (RIE) system, and 3) rapid thermal annealing (RTA) at 550 °C in O<SUB>2</SUB> ambient. For post-deposition treatments, samples are annealed at different temperatures under O<SUB>2</SUB> ambient in an RTA system after ohmic metallization and ALD of Al oxide.The results show that all three pre-treatments reduce J<SUB>REV</SUB> of GaN-based MOS diodes at −10 V by one to several orders of magnitude and increased turn-on voltage (V<SUB>on</SUB>) of the diodes (3 V to 6 V). Post-deposition annealing improves V<SUB>on</SUB> if the annealing temperature is below 500 °C, while the reverse current is similar to Schottky diodes. However, significant degradation is observed in both forward and reverse current-voltage characteristics if the annealing temperature is higher than 500 °C. These results suggest that the leakage current path at low biases for untreated ALD Al oxide is at the oxide/AlGaN interface and this leakage current can be effectively suppressed by a pre-ALD plasma or thermal oxidation process.